In numerous applications, it is desirable to have laser pulses with some or all of the following features: high-peak power, short pulse duration, small pulse-to-pulse intensity fluctuation and shape variation, stable single transverse mode, single longitudinal mode without pulse-to-pulse mode competition, and a polarized output. Q-switching is a commonly used technique for generating short laser pulses with high peak power. The function of Q-switching is to prevent lasing in a laser oscillator by spoiling the Q-value of the cavity through introduction of high losses while the gain medium is being pumped and, when sufficient energy is stored in the medium, to abruptly restore the Q-value by reducing the cavity loss so that stored energy is released in a single giant pulse.
Depending on the method used to control the cavity Q-value, there are two types of Q-switching, i.e. active or passive Q-switching. Active Q-switching requires an electrical or electronic control element e.g. a rotating mirror, an acousto-optic modulator, or an electro-optic modulator. Passive Q-switching relies on the laser pulse itself to control the cavity's Q-value. Typically, saturable absorbing materials such as organic dyes, color centers, or semiconductor materials are inserted into the cavity for inducing Q-switching. These materials have higher absorbance when the laser intensity is low and smaller absorbance when the laser intensity is high. Thus the saturable absorber tends to promote pulsed operation by enhancing the peaks and suppressing the leading and trailing edges of the pulses. Since passive-Q-switching does not require an external control element, it is simpler to use.
U.S. Pat. 5,119,382 to Kennedy et al. show a passive Q-Switched laser system wherein the Q-switch is housed externally to the laser structure. Passive Q-switching can be further simplified if the gain medium includes saturable absorbers so that Q-switching can take place without external control. Self Q-switching in solid-state lasers is described by the article entitled "Compact GSGG;Cr:Nd laser with passive Q-switching" by A. A. Danilov, et al , Soviet Journal of Quantum Electronics Volume 17(5), pages 573-574. The article discloses a flashlamp-pumped laser made of a gadolinium, scandium, gallium, garnet crystal (GSGG), doped with trivalent chromium, and neodymium.
The Danilov et al. self Q-switched laser has the advantage of simplicity and compactness, but it is still a conventional flashlamp pumped laser with external mirrors and thus is not immune to the common problems of Q-switched lasers such as intensity fluctuation, thermally-induced depolarization, and multi-longitudinal modes. Danilov et al. do not appear to have investigated the effects of a saturable absorber on lasing characteristics, such as frequency and polarization. In particular, the possibility of achieving longitudinal mode stabilization utilizing distributed saturable absorbers is not discussed. It has been found by the Inventors hereof that longitudinal mode stabilization is effective only when certain specific conditions are satisfied.
Single-longitudinal-mode operation is the most difficult to achieve. In a homogeneously broadened solid-state laser, single longitudinal mode operation is often spoiled by spatial hole burning (i.e. plural modes emerging from disparate locations with respect to the laser's transmitting axis). In a conventional Q-switched solid-state laser, a number of longitudinal modes operate simultaneously, and beating among these longitudinal modes results in a spiky pattern superimposed on the temporal envelope of the pulses.
To maintain a single longitudinal mode, it is necessary to eliminate spatial hole burning, for example, by using a ring cavity. One prior art method for eliminating spatial hole burning in a standing-wave resonator involves the use of an additional wavelength selective element, such as an etalon. However, the use of etalon both adds to the complexity of the laser device and still does not completely eliminate the pulse-to-pulse hopping, especially when the laser is subject to vibrations.
Another prior art method for ensuring single longitudinal mode operation is the use of a very short laser cavity, referred to as microchip cavity. Typically, single longitudinal-mode operation can be obtained if the free spectral range of the cavity is larger than the bandwidth of the gain spectrum. For example, in order to obtain a single longitudinal mode in a neodymium-doped, yttrium, aluminum, garnet (Nd:YAG) laser with 180 GHz bandwidth, the laser cavity must be less than 0.7 mm. Such a short cavity has the drawback of having small pulse energy due to the small gain volume.
It is an object of this invention to provide a simple self Q-switched laser which can generate a single longitudinal mode without pulse-to-pulse mode competition, and can also provide both intensity and pulse shape stability.
It is a further object of this invention to provide a self Q-switched laser that is monolithic in structure.